Metabolic compartments in Dictyostelium discoideum.

Dictyostelium discoideum was exposed to low levels of ~-14C]glucose at various stages during the culmination process, and seven metabolites were isolated to determine their specific radioactivities. Steady state models having 15 metabolite pools were constructed to interpret the data obtained. The models are based on certain assumptions, the flux of key reactions, and the cellular concentrations of metabolites; they simulate the specific radioactivities of the metabolites isolated. These analyses have led to the following conclusions. 1) Metabolically, two cell types can be distinguished during sorocarp construction: presumptive spore and stalk cells and mature stalk cells. 2) There are separate pools of glucose and hexose phosphates in the stalk cells and in the presumptive spore and stalk cells. The pools of UDP-glucose in the presumptive cell types have comparable specific radioactivities; this metabolite and the end product saccharides are not synthesized in stalk cells. 3) After the glycogen and end product saccharides are synthesized in presumptive spore and stalk cells, they are distributed into spore or stalk cells where they are accumulated (cellulose, cell wall glycogen), degraded (glycogen), or both (trehalose). 4) As glycogen levels fall during culmination, the amount which turns over is constant, thereby increasing from 3 to 9%. 5) Net glycogen degradation occurs in both cell types; unlabeled glycogen must be degraded in stalk cells. 6) In young sorocarps, stalk cells are approximately 10 times more permeable to exogenous glucose than presumptive stalk and spore cells. 7) Metabolically, the presumptive cell types and stalk cells behave like two independent cell populations; stalk metabolites are not used as precursor material by prespore and prestalk cells. Many simulation studies were carried out in which fluxes or the relative pool sizes not fixed by the data were varied singly or together. It was found that the models are highly constrained by the extensive data available; relatively few compensatory effects are possible among the parameters involved.